Coupling connection for slot antenna



Jan. 28, 1958 D. w; BLANCHER 2,821,708

COUPLING CONNECTION FOR SLOT ANTENNA Filed June 1. 1954 INVENTOR. D. 14 Blane/rev ATTORNEY United States Patent COUPLING CONNECTION FOR SLOT ANTENNA Donald W. Blancher, Van Nuys, Calif., assignor to Bendix Aviation Corporation, North Hollywood, Calif., a corporation of Delaware Application June 1, 1954, Serial No. 433,725

5 Claims. (Cl. 343-767) This invention relates to slot antennas and to apparatus for electrically coupling them to transmission lines.

,l atented Jan. 28, 1958 frequencies of operation of such devices, only that portion the slot 11 by potential developed between the opposite An object of the invention is to provide a simple prac- I ticable and effective coupling structure for delivering electrical energy between a coaxial transmission line and a slot antenna.

Another object is to provide a very compact coupling structure capable of matching the impedance of a coaxial transmission line to that of a relatively high voltage point on a slot antenna.

Another object is to provide a simple and compact coupling structure for coupling a coaxial transmission line, which is unbalanced to ground, to a slot antenna that is balanced to ground.

Other more specific objects and features of the invention will appear in the description to follow.

Slot antennas have been known for many years and have obvious mechanical advantages over the conventional antennas in certain applications, such as on high-speed aircraft where the slot can be placed in the wall or skin of the aircraft without protruding structures that would alter the aerodynamics configuration of the craft. However, use of slot antennas in the VHF spectrum has been limited because of the problem of providing an efficient feed mechanism therefor. The problem is due in large measure to the difiiculty of connecting an unbalanced transmission line, such as a coaxial cable, of relatively low impedance to a balanced slot antenna in such a way as to secure a good impedance match.

This problem is solved in a satisfactory manner in accordance with the present invention by employing as a coupling element between a coaxial cable and a slot antenna an L-shaped conductor positioned in the slot adjacent one end thereof with the long leg parallel to and closer to one wall of the slot than the other, and connected at its free end to the center (high potential) conwalls of the slot to cause radiation of electromagnetic energy in a well-known manner. Contrariwise, received electromagnetic waves produce an electric field between the walls of the slot, which produces a potential therebetween. For convenience, the invention will be described as applied to a transmitter.

For eflicient operation, the slot 11 should have an electrical length of one-half wave at its frequency of operation. This result may be obtained either by making the slot exactly equal to a half wave length, or by making it shorter and providing auxiliary tuning, as by means of a condenser 12 connected across the slot at its midpoint. Either type of slot antenna may be employed with the present invention, although the latter is particularly use- 7 ful with tuned slots of length less than one-half wave in physical dimension because of the increased ditficulty of feeding such slots by other known methods.

Referring now to Fig. 2, there is shown a coaxial cable 13 having an outer conductor 14 which is terminated'at the left end of the slot 11 and is mechanically attached to the wall 10 by a clamp 15. The cable 13 has an inner conductor 16 which is insulated from the outer conductor 14 and will sometimes be referred to as the high potential conductor, since, as is customary in a coaxial cable, the outer conductor 14 is at ground potential. This conductor 16 is extended into the slot 11 and connected to the free end of the long log 18 of an L-shaped coupling'conductor 17. The leg 18 is a fiat strip and extends parallel to the sides 20 and 21 of the slot 11 and substantially closer to one wall 20 than to the other wall 21. The short leg 19 ductor of the coaxial line, the short leg of the conductor 7 extending across the slot into electrical contact with the other wall of the slot. For reasons that will be more fully explained hereinafter, this exceedingly simple and compact coupling structure functions efliciently to transfer electric energy between the coaxial cable and the slot antenna.

In the drawing:

Fig. 1 is a face view of the inner side of a portion of a wall defining a slot antenna having a coupling structure in accordance with the invention.

Fig. 2 is an enlarged view of a portion of the wall at the feed end of the slot, showing a portion of the coupling structure in section.

Fig. 2a is a cross section in the plane A-A of Fig. 2.

Fig. 3 is a schematic diagram showing the equivalent electrical circuit of the coupling structure.

Referring first to Fig. 1, there is shown a metal wall 19 having a slot 11 therein and constituting the essential structure of a slot antenna. It is to be understood that the wall 10 can be of unlimited extent and constitute the outer wall or skin of an aircraft or the like. At the high of the L-shaped conductor 17 extends transversely across the slot away from the wall 20 and into contact with the wall 21 to which it is bonded.

To provide mechanical support for the conductor 16 and the member 17, the portion of the slot 11 in which the structure 17 is positioned may be filled with a suitable solid dielectric material 22 in which the conductor 17 is imbedded. This dielectric material 22 preferably has a higher dielectric value than air, so as to increase the capacity between the long leg 18 and the sides 20 and 21 of the slot.

The equivalent circuit of the arrangement of Fig. 2 is shown in Fig. 3, in which the side 20 of the slot is shown as a conductor having distributed inductance L 0, the side 21 of the slot is shown as a conductor having distributed inductanc L and the long leg 18 of the condoctor 17 is shown as a conductor having distributed inductance L The distributed capacity between the leg 18 and the side wall 20 is illustrated as capacities C20, and C and the distributed capacity between the leg 18 and the side wall 20 is represented as capacities C The short leg 19 is represented as a conductor having distributed inductance L The radiation resistance of the antenna as seen by the coupling mechanism is represented by a resistor R connected between the conductors 20 and 21. It will be observed that the left ends of the conductors 20 and 21 are shown connected to the ground conductor 14 of the coaxial cable, so that they are at ground potential at that point.

It will be observed from inspection of Fig. 3 that the lines 18 and 21 constitute one transmission line having series inductance L and L and distributed capacity C and terminating in the inductance L19, whereas the conductor 18 and the conductor 21) define a transmission line containing series inductance L and L and shunt capacity C and terminated by the capacity C It is to be noted that because of the closer positioning of the leg 18 (Fig. 2) to the wall 20 than to the wall 21, it has a substantially greater capacity to wall 20 than to wall 21. In Fig. 3, the capacity C represents the difference between the total capacity between conductor 17 and slot wall 20 and the capacity between conductor 17 and the slot wall 21. The two transmission lines recited above are fed in parallel from the coaxial cable 13. The leg 18 and the side walls 20 and 21 of the slot define segments of uniform transmission lines between the end of the slot and the short leg 19 of member 17, but at that point they are terminated by impedances of opposite reactance. Thus, the segment consisting of the conductor 18 and the side walls 21 are terminated by the inductance L whereas the segment consisting of the conductor 18 and the slot wall 20 are terminated by the capacity C The potential across the inductance L leads the current in the circuit by approximately 90, and the potential across the capacity C lags the current in the circuit by approximately 90", so that the alternating voltages at the extremities of inductance L and capacity C are approximately 180 out of phase. Hence, the two sides of the slot are excited with potentials of opposite polarity and in balanced condition with respect to ground.

Inasmuch as the long section 18 of the conductor 17 and the sides 20 and 21 of the slot form segments of transmission lines that are terminated at their right ends in impedances that are different from their surge impedances, the segments act as voltage transformers, and the impedance of the transmission line rises at the same rate as the impedance rises between the sides of the slot. Therefore, the coupling element provides an automatic impedance match to the slot.

In a typical installation for operation at 225 megacycles, the structure shown in Figs. 1 and 2 may have the following values:

Length of slot 11 in 8 Width of slot 11 in Approximate value of condenser 12 mmf 17 Length of leg 18 in 1 Width of leg 18 in /8 Area of leg 18 juxtaposed to the sides of slot sq. in-.. Spacing of leg 18 from side 20 in ,4, Spacing of leg 18 from side 21 ..in Dielectric coefiicient of dielectric 22 2.1 Surge impedance of coaxial cable 13 ohm 50 and tuned by added capacity such as the condenser 12, rises rather abruptly a short distance from the end and thereafter levels olr somewhat, the length of the matching conductor becoming non-critical after the high impedance region of the slot is reached. The length of the matching conductor is affected to some degree by the dielectric material between the slot and conductor (increase of dielectric constant appears to allow shorter conductors to be used). A practical ratio of matching conductor length to slot length appears to be 1:8. The ratio of conductor to slot length may vary if the slot width and depth are changed.

Although for the purpose of explaining the invention, a particular embodiment thereof has been shown and described, obvious modifications will occur to a person skilled in the art, and I do not desire to be limited to the exact details shown and described.

I claim:

1. In combination: a slot antenna consisting of a conductive wall member defining an elongated slot closed at its ends and having juxtaposed side walls; a transmission line terminating adjacent one end of said slot and having a grounded conductor connected to said wall member, and having a high potential conductor; and a coupling conductor in said slot electrically connecting said high potential conductor to one wall of said slot at a point spaced from said one end thereof, said coupling conductor comprising a longitudinal section having one end adjacent said one end of said slot and connected to said high potential conductor and having a transverse section extending from the other end of said longitudinal section to said one wall of said slot, said longitudinal section being positioned closer to the other wall than to said one wall, whereby its capactive coupling to said other wall is greater than its capacitive coupling to said one wall.

2. Apparatus according to claim 1 including dielectric material of higher dielectric constant than air interposed between said longitudinal section and said other wall.

3. Apparatus according to claim 1 in which said longitudinal section extends longitudinally parallel to said walls.

4. Apparatus according to claim 1 in which said longitudinal section is of ribbon form extending laterally parallel to said walls.

5. Apparatus according to claim 4 in which the width of said longitudinal section is substantially the same as the width of the walls of the slot.

References Cited in the file of this patent UNITED STATES PATENTS 2,629,051 Lindenblad Feb. 17, 1953 2,654,842 Engelmann Oct. 6, 1953 2,660,674 Brown NOV. 24, 1953 2,755,465 Ramsay July 17, 1956 FOREIGN PATENTS 664,150 Great Britain Jan. 2, 1952 

